The present invention relates to a lithium secondary battery that has large capacity and remarkable safety, especially to a lithium secondary battery that can suppress short circuits caused by the generation of dendrites from the negative electrode, that has high energy density, and that is excellent in charge and discharge-cycle performance.
Lithium secondary batteries with an organic electrolysis solution have been widely used. Their attractive feature being their high energy output per unit volume or unit weight in comparison with other batteries. In exploiting this advantage, researchers and engineers have been advancing the development and practical applications of the batteries as a power source for mobile communication devices, notebook-type personal computers, and electric vehicles.
The types of lithium secondary batteries include an organic electrolysis solution type, in which an organic electrolysis solution is impregnated in a porous polymer separator, and a gel polymer type, in which a gelatinous polymer. containing a large amount of electrolysis solution is used.
Both the organic electrolysis solution type and the gel polymer type have a substantial amount of electrolysis solution, thus posing problems. The problems include a poor withstanding property against voltage, instability against the electrode material (especially against carbon which is usually used as the negative electrode), and gas evolution. In addition, these organic electrolysis solutions are intrinsically inflammable substances, and therefore a short circuit resulting from a temperature rise or shock for any reason may cause an explosion.
Moreover, organic electrolysis solution-type and gel polymer-type batteries have been required to increase the energy density as their important technical challenge. Their limit at the present is about 300 Wh/l, and it is strongly required to increase this limit to 400 Wh/l or more. Researchers and engineers have been studying the use of metallic lithium as the negative electrode in order to solve the foregoing problems and improve the properties effectively.
However, when a lithium-containing material is used as the negative electrode, the electrolytic layer is affected by the change in the thickness of the metallic lithium utilized in charging and discharging and by the change in the shape of the negative electrode at the time of charge and discharge. It is particularly notable at increased cycles, such as several hundred cycles or more. Metallic lithium has a strong tendency to react with water vapor in the air, so that it is necessary to provide a device to block the air in the filming process.
Moreover, with a lithium secondary battery containing an organic electrolysis solution, repetition of charge and discharge causes dendritic metallic lithium to grow on the surface of the metallic lithium. This may cause an internal short circuit between the electrodes, triggering explosion and other abnormalities.
To eliminate this possibility, the following techniques have been proposed:
1. Formation of a compound layer by treating the surface of the metallic lithium to be used as the negative electrode. The types of compound layers include a polymer layer, a fluoride layer, a carbonic compound layer, and an oxide layer.
2. Production of an entirely solid battery containing no electrolysis solution that may cause explosion. For example, an organic polymer or inorganic crystals can be used as the electrolyte.
The foregoing techniques, however, have the following problems:
1-3. It is known that the techniques for the surface treatment of the metallic lithium include the following:
(a) The surface treatment is performed before forming the battery.
(b) A compound layer is formed by spontaneously reacting the metallic lithium with the compounds in the electrolysis solution and in the material for the positive electrode when the battery is formed.
1-2. In (a) above, it is understood that the acid treatment or plasma treatment forms a lithium fluoride, a lithium carbonate, or a lithium oxide layer, each of which is effective in suppressing the growth of the lithium dendrite at the time of charge and discharge. With this technique, however, repetition of charge and discharge poses problems such as the formation of voids at the interface, separation of the compound layer, and concentrated growth of metallic lithium in cracks and pinholes in the compound layer.
1-3. In (b) above, because substances that form a compound layer by reacting with metallic lithium are added in the organic electrolysis solution, a compound layer is formed continually at the interface on condition that the metallic lithium comes into contact with the electrolysis solution. As a result, although problems such as separation can be avoided with high possibility, impurities inevitably contained in the organic electrolysis solution cause the compound layer formed on the surface of the metallic lithium to be nonuniform. This reduces the effectiveness in suppressing the growth of the dendrites of metallic lithium.
2-1. The entirely solid type has a solid electrolyte. This poses a problem in the contact between the electrode and the electrolyte. The resultant reduction in the contact area increases the contact resistance, preventing the extraction of a large amount of the electric current.
2-2. The difficulty in handling the solid electrolyte restricts application forms. The types of materials as the solid electrolyte include a sulfide family, an oxide family, a nitride family, and a mixture of these, such as an oxynitride family and an oxysulfide family. However, although a compound containing sulfide has high lithium-ion conductivity, it has drawbacks such as high hygroscopic property and high hydrolytic property at the same time. These drawbacks cause the electrolytic layer to be difficult to handle after it is formed. More specifically, the electrolytic layer requires to be sealed in an inert gas atmosphere during the formation of the battery and transportation. It also requires provision of a glove box. These requirements pose problems in productivity and cost.
2-3. Lithium ion-conductive solid electrolytes mainly have been studied as a bulk-shaped sintered body or a powder for practical use. This restricts application forms, reduces total ionic conductivity, and lowers battery performance. On the other hand, when a thin-film electrolyte is used, it is difficult to suppress the formation of pinholes and cracks. In particular, when the positive electrode contains an organic electrolysis solution, the electrolysis solution effusing from the positive electrode penetrates through the pinholes and cracks to reach the surface of the negative electrode. Then, the electrolysis solution reacts with the negative electrode to cause concentrated growth of dendrites at the pinholes and cracks. This may cause a problem of short circuiting between the electrodes. In addition, when the current capacity per unit area is increased, the stress caused by the volume change of the negative electrode at the time of charge and discharge may fracture the electrolytic layer.
Consequently, the main object of the present invention is to offer a lithium secondary battery that can suppress short circuits caused by the generation of dendrites from the negative electrode, that has high energy density, and that is excellent in charge and discharge-cycle performance.
The present invention achieves the foregoing object by providing an electrolytic layer that is formed by an inorganic solid electrolyte and a positive electrode that contains an organic high polymer. This constitution prevents the growth of dendrites on the metallic lithium at the time of charge and discharge, suppresses the reaction between the organic electrolysis solution and the negative electrode, and suppresses the temperature rise in the battery even at the time of over charging. In short, an explosion can be avoided. The following is a detailed explanation of the electrolytic layer, positive electrode, negative electrode, and battery constitution. These conditions can be utilized separately or in combination.
FIG. 1 is an enlarged cross section of the main portion of the lithium secondary battery produced in Example 4-1. The battery comprises a collector 1, a positive electrode 2, a separator 3, an inorganic solid electrolytic layer 4, a negative electrode 5, and a collector 6.
 less than Material greater than 
It is effective to use an inorganic solid electrolyte as the electrolytic layer. This is because an inorganic solid electrolyte forms an interfacial layer having a graded structure composition at the interface with the metallic lithium. Whereas an organic polymer produces a definite interface between the metallic lithium and the organic polymer layer, an inorganic solid electrolyte forms at the interface a layer in which the metallic lithium and .th lithium-containing inorganic compound are mixed, thereby preventing the separation.
Examples of the inorganic solid electrolytes include a sulfide family, an oxide family, a nitride family, and a mixture of these, such as an oxynitride family and an oxysulfide family. The types of the sulfides include Li2S and compounds of Li2S and SiS2, GeS2, or Ga2S3. The types of the oxynitrides include Li3PO4-xN2x/3, Li4SiO4-xN2x/3, Li4GeO4-xN2x/3 (where 0 less than X less than 4), and Li3BO3-xN2x/3 (where 0 less than X less than 3).
In particular, when sulfur is contained, the interfacial layer having a graded structure in composition can be easily formed at the surface of the metallic lithium. The formation of this interfacial layer can prevent the separation of the inorganic solid electrolytic layer caused by the penetration of the organic electrolysis solution into the gap at the interface between the metallic lithium and the inorganic solid electrolytic layer when the metallic lithium deposits and dissolves at the negative electrode at the time of charge and discharge.
Furthermore, the present inventors found that when at least either oxygen or nitrogen is contained in addition to the sulfur, the foregoing effect is intensified. The reason is that oxygen or nitrogen reacts strongly with the metallic lithium to further increase the bonding between the inorganic solid electrolytic layer and the metallic lithium. This addition of oxygen or nitrogen also materializes an ionic conductivity as high as 10xe2x88x923 to 10xe2x88x922 S/cm. This is attributable to the effect of the difference in polarity between the constituting elements and of the generation of distortion in the network structure, which distortion produces interstices in which lithium ions can readily travel. This addition is also effective in suppressing the high hygroscopic property, one of the drawbacks of these materials, especially the oxysulfide family.
It is desirable that the inorganic solid electrolytic layer contain an elementary lithium of not less than 30 atm. % and not more than 65 atm. %. If less than 30 atm. %, the ionic conductivity decreases, thereby giving the layer high resistance. This low content also deteriorates the bonding quality between the inorganic solid electrolytic layer and the metallic-lithium layer. If more than 65 atm. %, although the bonding quality between the inorganic solid electrolytic layer and the metallic-lithium layer is improved, the inorganic solid electrolytic layer becomes polycrystallized and porous, making it difficult to form a dense, continuous film of the inorganic solid electrolyte. In addition to that, this high content produces electronic conductivity to cause internal short circuits when the battery is formed. The result is deterioration of the battery performance. Therefore, it is desirable that the electrolytic layer be amorphous.
It is desirable that the inorganic solid electrolyte contain in addition to the lithium at least one type of element selected from the group consisting of phosphorus, silicon, boron, aluminum, germanium, and gallium (hereinafter, these elements are called xe2x80x9cadditional elementsxe2x80x9d) and also contain sulfur. As mentioned previously, it is effective if the inorganic solid electrolyte is amorphous. These xe2x80x9cadditional elementsxe2x80x9d can form this amorphous framework by constituting a network structure through sulfur and supply sites with a size suitable for the lithium ions to travel. The xe2x80x9cadditional elementsxe2x80x9d can also give negative charges suitable in amount in order for sulfur atoms at the ends of the amorphous framework to trap positively charged lithium ions. In other words, the negatively charged sulfur atoms at the ends trap positively charged lithium ions moderately and mildly so that the lithium ions can travel without being undesirably firmly fixed.
As described previously, at least either oxygen or nitrogen can be added to the inorganic solid electrolyte in addition to the lithium, xe2x80x9cadditional elements,xe2x80x9d and sulfur. The oxygen or nitrogen can further increase the lithium-ion conductivity. This is attributable to the effectiveness of the oxygen or nitrogen atoms widening the interstices in the formed amorphous framework, thus alleviating the interference of the movement of the lithium ions.
Another effect of the xe2x80x9cadditional elementsxe2x80x9d contained in the inorganic solid electrolyte is the improvement in bonding quality between the inorganic solid electrolytic layer and the metallic lithium. The xe2x80x9cadditional elementsxe2x80x9d further increases an affinity between the inorganic solid electrolyte and the metallic lithium. As described before, the addition of lithium, sulfur, oxygen, and nitrogen improves the bonding quality between the inorganic solid electrolytic layer and the metallic lithium. However, the addition of elements other than the foregoing four types of elements and the xe2x80x9cadditional elementsxe2x80x9d deteriorates the affinity between the inorganic solid electrolytic layer and the metallic lithium, so that the inorganic solid electrolytic layer tends to separate easily from the metallic lithium.
 less than Ionic Conductivity greater than 
The present inventors found the following ionic conductivity-related mechanism: Ionic conductivity is an important property of the constituent materials of the electrolytic layer. Nevertheless, with the conventional techniques, the compound layers formed on the metallic lithium have an ionic conductivity no more than 10xe2x88x927 S/cm at room temperature. This low conductivity permits an organic electrolysis solution that has an ionic conductivity of the order of 10xe2x88x923S/cm to penetrate into the interface between the compound layer and the metallic lithium through unavoidably existing pinholes and cracks even when the compound layer has a thickness no more than several nanometers. Consequently, the lithium ions tend to flow through the highly ion-conductive organic electrolysis solution. As a result, the interface between the compound layer and the metallic lithium suffers erosion, so that the compound layer easily separates from the metallic lithium. The result is a reduction in the covering effect.
To avoid the foregoing mechanism, the present invention form a highly ion-conductive electrolytic layer in order for the lithium ions to flow mainly through the electrolytic layer. It is desirable that the electrolytic layer have a lithium-ion conductivity of 10xe2x88x925 S/cm or more at 25xc2x0 C. Even when the electrolytic layer (a film) has pinholes and cracks, the metallic lithium in the pinholes and cracks react with carbon dioxide ions, oxygen gas, water molecules, or fluorine ions that are contained in the electrolysis solution as unavoidable impurities, to form on the surface of the metallic lithium a low ion-conductive layer, such as a lithium carbonate, lithium oxide, or lithium fluoride layer. This low ion-conductive layer protects the pinholes and cracks to suppress the growth of dendrites and to cause the lithium ions to flow mainly through the electrolytic layer. It is more desirable that the inorganic solid electrolytic layer have a lithium-ion conductivity of 5xc3x9710xe2x88x924 S/cm or more at 25xc2x0 C., which is 10% or more of the ionic conductivity of the organic electrolysis solution. It is preferable that the inorganic solid electrolytic layer have a lithium-ion conductivity of 1xc3x97103 S/cm or more at 25xc2x0 C.
In addition, it is desirable that at least one of the following measures be incorporated into the formation of a battery in order to effectively form a low ion-conductive compound in combination with metallic lithium:
(a) to intentionally add in advance to the organic electrolysis solution carbon dioxide, halides, an anionically polymerizing organic monomer, or an organic molecule that forms a compound with lithium;
(b) to use as the electrolytic salt (a solute) for the organic electrolysis solution imide-family organic lithium or another material that permits fluorine compounds to dissolve easily; and
(c) to use as the positive-electrode material a disulfide-family organic material or another material that permits sulfur compounds to dissolve in the organic electrolysis solution.
 less than Dual-Layer Structure greater than 
The dual-layer structure of the foregoing inorganic solid electrolytic layer further facilitates its handling. Although lithium-ion conductive compounds containing sulfide as the material for the electrolytic layer have high lithiumion conductivity, they have the drawback of having simultaneously a high hygroscopic property and high hydrolytic property, as described previously. On the other hand, although lithium-ion conductive compounds containing oxide have chemical stability against the air, they are unstable against low ion-conductive compounds and the metallic lithium in the negative electrode. Consequently, when an electrolytic layer consists of two layers (a negative electrode-side layer and a positive electrode-side layer) in which the negative electrode-side layer is a film made of a lithium-ion conductive compound containing sulfide (lithium sulfide or silicon sulfide) and the positive electrode-side layer is a film made of a lithium-ion conductive compound containing oxide, the electrolytic layer can be stable against the air and have high ionic conductivity.
The positive electrode-side layer acts as a protective film that prevents the reaction with water vapor while the electrolytic layer is in the air and dissolves into the organic electrolysis solution when incorporated into a battery. Moreover, dissolved constituent elements of the positive electrode-side layer form a low ion-conductive layer by reacting in the pinholes and cracks in the electrolytic layer with the metallic lithium to prevent the concentrated growth of dendrites.
It is effective for the positive electrode-side layer as the protective film to be a lithium-ion conductive body that contains not only phosphorous but also at least either oxygen or nitrogen. Specifically, a phosphate or phosphorus oxynitride compound is suitable.
It is effective for the positive electrode-side layer to contain an Li constituent not less than 30 atm. % and not more than 50 atm. %. If less than 30 atm. %, a strong possibility of imperfect dissolution may result. If more than 50 atm. %, the hygroscopic property emerges so that the layer cannot act as the protective film.
It is desirable that the positive electrode-side layer be thin. However, if it is excessively thin, the negative electrode-side layer containing sulfides cannot be effectively isolated from the air. It is desirable that the positive electrode-side layer have a thickness not less than 10 nm or not less than 1% of the thickness of the negative electrode-side layer. If the positive electrode-side layer is excessively thick, it becomes difficult to maintain the high ionic conductivity and to dissolve in the electrolysis solution. Specifically, it is desirable that the positive electrode-side layer have a thickness not more than 25 xcexcm or not more than 50% of the thickness of the negative electrode-side layer. In particular, when the negative electrode-side layer is a film made of lithium ion-conductive compounds containing sulfide and the positive electrode-side layer is a film made of lithium ion-conductive compounds containing oxide, it is desirable that the positive electrode-side layer have a thickness not less than 0.1 xcexcm and not more than 2 xcexcm in terms of the battery performance.
 less than Thickness greater than 
It is desirable that the electrolytic layer have a total thickness not less than 50 nm and not more than 50 xcexcm. If the thickness is more than 50 xcexcm, although the covering effect is further increased, the ionic conductivity is decreased to deteriorate the battery performance. Moreover, the time and energy needed for the formation of the film increase excessively to cause the film to be unpractical. In particular, the resistance of the electrolytic layer against the ionic conduction increases, posing a problem in that the output current cannot be sufficiently increased. If the thickness is less than 50 nm, the electron-conductive component increases, posing a problem in that it tends to cause self-discharging. In addition, it becomes difficult to suppress the formation of pinholes in the thin-film electrolyte. As a result, when the positive electrode contains an organic electrolysis solution, the electrolysis solution effusing from the positive electrode penetrates through the pinholes to reach the surface of the negative electrode. Then, the electrolysis solution reacts with the negative electrode to cause dendrites to form, posing another problem.
In particular, when the foregoing dual-layer structure is employed, it is desirable that the electrolytic layer have a total thickness not less than 2 xcexcm and not more than 22 xcexcm. If the thickness is less than 2 xcexcm, it becomes difficult to suppress the formation of pinholes and cracks in the thin-film electrolyte. As a result, when the positive electrode contains an organic electrolysis solution, the electrolysis solution effusing from the positive electrode penetrates through the pinholes and cracks to reach the surface of the negative electrode. Then, the electrolysis solution reacts with the negative electrode to cause dendrites to form, causing a short circuit across the electrodes. In addition, when the current capacity per unit area is increased, the stress caused by the volume change of the negative electrode at the time of charge and discharge may fracture the electrolytic layer. If the thickness is more than 22 xcexcm, the resistance of the electrolytic layer against the ionic conduction increases, posing a problem in that the current capacity per unit area cannot be sufficiently increased, so that the efficiency is decreased.
 less than Material greater than 
It is desirable that the material for the positive electrode be made of an organic high-polymeric binder that contains an active material. It is desirable that the binder be at least one type selected from the group consisting of polyacrylonitrile, polyethylene oxide, and polyvinylidene fluoride, all of which contain an organic solvent such as ethylene carbonate, propylene carbonate, or dimethyl carbonate. It is desirable that the active material be at least one type selected from the group consisting of LixCoO2,LixMn2O4, and LixNiO2 (where 0  less than X less than 1). In addition, It is desirable that a carbon powder be mixed into the high polymer in order to give electronic conductivity.
The organic high polymer in the material for the positive electrode may be a disulfide-family high polymer or polypyrrole-family material all of which contain polyaniline that has not only ionic conductivity but also electronic conductivity.
Regardless of a high polymer selected from the foregoing group, it is important to add either LiPF6 or LiCF3SO3, a lithium salt. This addition gives good contact between the electrolytic layer and the positive electrode. This good contact considerably reduces the interfacial resistance between the inorganic solid electrolytic layer and the positive electrode (the interfacial resistance has been a serious problem in the case of an inorganic solid electrolytic layer). As a result, the output current can be increased. Moreover, the following drawbacks are significantly improved (these drawbacks have also been a serious problem): gas evolution, a development of over voltage at the time of charging caused by the high internal resistance, and considerable deterioration in battery performance when left under charged condition.
When a powder of an inorganic solid electrolyte having lithium-ion conductivity is added to the positive electrode, the amount of the organic electrolysis solution can be further reduced, thereby lessening the problems caused by the organic electrolysis solution. It is desirable that the inorganic solid electrolyte be the foregoing highly ion-conductive material. Nevertheless, any material having an ionic conductivity of 10xe2x88x923 S/cm or more may be used.
 less than Organic Electrolysis Solution in Positive Electrode greater than 
It is difficult to eliminate an organic electrolysis solution completely in terms of the practical aspect of battery performance. However, it is possible to obtain a high-performance battery by combining the following measures:
(a) an organic electrolysis solution is contained mainly around the active material in the positive electrode;
(b) metallic lithium is used as the negative electrode; and
(c) an inorganic film having lithium-ion conductivity is formed on the negative electrode.
This type of lithium secondary battery has the following advantages:
(a) reduction in the amount of the organic electrolysis solution;
(b) suppression of the growth of metallic-lithium dendrites on the negative electrode;
(c) prevention of the contact with the positive electrode and suppression of the reaction with the electrolysis solution, both resulting from the covering effect on the surface of the negative electrode.
The mechanism of gas evolution caused by the organic electrolysis solution is yet to be clarified. Nonetheless, when an organic electrolysis solution is used, the battery constitution of the present invention can reduce the amount of the organic electrolysis solution to no more than 10% of the conventional amount. In this case, the present inventors also found that even when the battery is left under a charged condition, the considerable reduction in the battery performance caused by the decomposition and deterioration of the electrolysis solution can be significantly suppressed (this considerable performance deterioration phenomenon has been experienced in conventional batteries).
When pinholes and cracks are formed in the inorganic solid electrolytic layer, the metallic lithium grows concentratedly along these portions at the time of charging, readily resulting in an internal short circuit. Even when these pinholes and cracks are formed, however, the stabilized charge and discharge performance and safety can be achieved by adjusting the organic electrolysis solution contained in the positive electrode. This adjusting method is explained below.
First, the ionic conductivity of the organic electrolysis solution is reduced to a value equal to or lower than that of the inorganic solid electrolytic layer. This condition has the following effect: Even when pinholes and cracks are formed and the organic electrolysis solution penetrates into these portions to form ionic-conduction paths, Li ions travel mainly through the inorganic solid electrolytic layer that has higher ionic conductivity. As a result, the supply of Li ions into the pinholes and cracks is reduced, thereby suppressing the growth of metallic lithium. As a matter of course, an organic electrolysis solution that has a lower lithium-ion conductivity than that of the inorganic solid electrolytic layer may be used. As an alternative, the organic electrolysis solution in the positive electrode may come into contact with the lithium-containing material of the negative electrode in order to reduce the ionic conductivity of the organic electrolysis solution in the vicinity of the contact portion to a lower value than that of the inorganic solid electrolytic layer.
There are various methods to reduce the ionic conductivity of the organic electrolysis solution. For instance, the amount of the solute as the electrolyte may be reduced. A solvent that has high viscosity and remains low in ionic conductivity, such as sulfolane (tetrahydrothiophene 1,1-dioxide), may also be used.
Second, an organic electrolysis solution is used which contains an organic solvent that is reduced and decomposed when the organic electrolysis solution comes into contact with the metallic lithium. This condition has the following effect: When the organic solvent is reduced and decomposed, a part of it gasifies so that Li ion-travelling paths in the pinholes and cracks are blocked and, at the same time, the ionic conductivity is reduced. To be specific, a carboxylic ester family, such as methyl formate, is effective as the organic solvent.
Third, when an organic electrolysis solution comes into contact with metallic lithium, an organic solvent in the organic electrolysis solution polymerizes by the catalytic action or polymerization-starting action of the metallic lithium. Then, the polymer solidifies or becomes highly viscous to reduce the Li-ion conductivity. In addition, the mechanical action of the formed polymer or highly viscous body suppresses the growth of the metallic lithium. In this case, even if the inorganic solid electrolytic layer separates from the metallic lithium, the organic electrolysis solution effuses and the resultant high polymer or highly viscous body covers pinholes and cracks at the surface of the metallic lithium at all times. This enables the formation of an extremely safe battery.
The types of organic solvents that solidify or become highly viscous when they come into contact with metallic lithium include anionically polymerized monomers having olefinic linkage, such as styrene, acrylonitrile, methyl acrylate, butadiene, and isoprene. A material that contains the foregoing anionically polymerized monomer may also be used. In addition, a solvent that polymerizes by the action of metallic lithium to solidify or become highly viscous, such as acetonitryle, which has a nitryl group, may also be used as a part or as a whole.
 less than Material greater than 
The types of lithium-containing materials to be used as the negative electrode include not only metallic lithium itself but also lithium alloys. The types of lithium alloys include alloys with such elements as In, Ti, Zn, Bi, and Sn.
A layer of a metal that forms an alloy or intermetallic compound with lithium, such as Al, In, Bi, Zn, or Pb, may also be formed on the surface of the foregoing lithium-containing material. The negative electrode comprising the metal layer and the lithium-containing material enables the smooth movement of the metallic lithium at the time of charge and discharge, increasing the utilized thickness of the metallic lithium. It can also equalize the dimension change of the negative electrode at the time of charge and discharge, reducing the stress given to the electrolytic layer. This is attributable to the interface being stabilized with the electrolytic layer. When the negative electrode has a multilayer or a graded structure in composition, it achieves smooth movement of the metallic lithium and reduced stress given to the electrolytic layer. The metal such as Al, In, Bi, Zn, or Pb, which is relatively stable against the air, covers the negative electrode that acts as the substrate when the electrolytic layer is formed. This can stabilize the production and simplify the process.
The foregoing lithium-containing material may be used without any pretreatment when the electrolytic layer is formed. Generally, however, a metal that contains lithium has a thin oxide layer formed on its surface in many instances. Therefore, it is desirable to remove this oxide layer before forming a nitride or sulfide layer. This enables direct formation of the electrolytic layer onto the lithium-containing material, so that the contact resistance between the lithium-containing material and the inorganic solid electrolytic layer can be further reduced. The oxide layer can be removed by the argon plasma treatment. The nitride and sulfide can be formed by exposing the surface of the lithium-containing material to high-frequency plasma in an atmosphere of nitrogen gas or hydrogen sulfide. However, the formation is not limited to solely this method. The removal of an oxide layer and formation of a sulfide layer on the surface of the metallic lithium can also be achieved by heating the metallic lithium at the temperature of its melting point or higher after the metallic-lithium layer is formed.
 less than Surface Roughness greater than 
Surface roughness (Rmax) of the negative electrode, also, affects the battery performance considerably. It is desirable that the value of Rmax be not less than 0.01 xcexcm and not more than 5 xcexcm. If less than 0.01 xcexcm, good bonding with the electrolytic layer cannot be obtained, resulting in easy separation. In addition, smooth deposition and ionization of the metallic lithium may not be performed at the time of charge and discharge. It appears that the deposition and ionization are affected by the bonding with the electrolytic layer. If Rmax is more than 5 xcexcm, it becomes difficult to form a pinhole-free, dense electrolytic layer.
 less than Shape and Structure of Battery greater than 
A battery comprising the foregoing positive electrode, negative electrode, and electrolytic layer has a laminated structure in which the electrolytic layer is sandwiched between the positive and negative electrodes. The laminated body is housed in a battery case to be sealed. To elaborate further, first, the negative electrode-side collector and the negative electrode are bonded. An inorganic solid electrolytic layer without containing an organic electrolysis solution is formed on a lithium-containing material to be used as the negative electrode. Thus, a bonded body of the negative electrode and the electrolytic layer is produced. Second, a material containing an organic high polymer is formed on a positive electrode-side collector (copper or aluminum foil, for example) to obtain the positive electrode. Finally, the bonded body of the negative electrode and electrolytic layer is coupled with the positive electrode to complete a lithium secondary battery. This structure enables the reduction of contact resistance between the negative electrode and electrolytic layer and between the positive electrode and electrolytic layer, so that a favorable charge and discharge performance can be obtained. In addition to the laminated button-type battery described above, a laminated body of the negative electrode, electrolytic layer, and positive electrode may be rolled up to obtain a cylindrically shaped battery.
A separator may be placed between the positive electrode and the inorganic solid electrolytic layer. The material for the separator must have pores in which lithium ions can travel and to be stable without dissolving into an organic electrolysis solution. For instance, a nonwoven fabric or porous body formed of polypropylene, polyethylene, fluororesin, polyamide resin and so on may be used. A metal oxide film having pores may also be used.
It is not always required to provide a lithium-containing material as the negative electrode in advance. An inorganic solid electrolytic layer may be formed directly on the negative electrode-side collector to obtain a lithium secondary battery having sufficient performance. This is because the positive electrode contains a sufficient amount of lithium so that metallic lithium can be stored between the negative electrode-side collector and the inorganic solid electrolytic layer at the time of charging.